1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for exciting magnetic resonance in a specific portion of an object to be examined, which is placed in a static magnetic field, by applying a gradient magnetic field and high-frequency pulses (RF pulses) to the object, acquiring magnetic resonance (MR) echo signal data produced by magnetic resonance, and imaging the specific portion by using the MR echo signal data in accordance with a predetermined image reconstruction method. In particular, this invention relates to a magnetic resonance imaging (MRI) system and method employing a multiecho method in which a first MR echo signal is acquired by a spin echo (SE) method and then second and following MR echo signals are acquired by repeatedly applying 180.degree. pulses, wherein motion artifact due to the second and following MR echo signals can be prevented.
2. Description of the Related Art
In a general medical MRI system, a gradient magnetic field and a high-frequency pulse are applied to an object to be examined, which is placed in a static magnetic field, in accordance with a predetermined sequence for magnetic resonance (MR) excitation/magnetic resonance (MR) data acquisition. As a result, an MR phenomenon occurs in a specific portion of the object. As a result, an MR phenomenon occurs in a specific portion of the object. An MR signal induced by the MR phenomenon is detected. By subjecting the acquired MR data to imaging data processing including image reconstruction, the anatomical data or quality data of the specific portion of the object is imaged.
A conventional MRI system of this type generally comprises a static field generator, X-, Y-, and Z-axis gradient field generators, and high-frequency transmitter and receiver. When the X-, Y-, and Z-axis gradient field generators and the high-frequency transmitter are driven in accordance with a predetermined sequence, X-, Y-, and Z-axis gradient fields Gx, Gy and Gz and a high-frequency pulse are generated with a predetermined sequence pattern. As a result, an MR signal is generated and received by the receiver. The reception data is then subjected to predetermined image processing including image reconstruction. The tomographic image of a certain slice portion of an object to be examined is generated in this manner, and is displayed on a monitor.
In the sequence of MR excitation/MR data acquisition, the X-, Y-, and Z-axis gradient fields Gx, Gy and Gz are respectively used as, for example, a read gradient field Gr, and encoding gradient field Ge, and a slicing gradient field Gs.
As a conventional MR imaging method often used in such a system, there is known an imaging method based on a spin echo (SE) method using high-frequency pulses of 90.degree. pulse-180.degree. pulse sequence. In the spin echo method, data can be acquired by a multiecho sequence in which a single operation of magnetic resonance excitation allows successive generation of a plurality of magnetic resonance echoes. The SE method is often used for MR data acquisition using the multiecho sequence.
With reference to FIG. 1, a two-echo acquisition sequence for acquiring only a first echo (first MR echo) and a second echo (second MR echo) by using a multiecho method will now be described as an example of MR data acquisition using a multiecho sequence in a conventional SE method. FIG. 1 shows a sequence in a single encoding step.
In a first phase P11 of the sequence, a 90.degree. pulse and a slicing gradient field Gs are applied to an object to be examined to excite a predetermined slice of the object (the magnetization vector in the slice is flipped by 90.degree.). Thereafter, an encoding gradient field G2 having an amplitude corresponding to an encoding step is applied to the object, and then a 180.degree. pulse is applied to the object to refocus a spin phase. After a time period TE (echo time) from the application of the 90.degree. pulse, a first echo signal is acquired, while a read gradient field Gr is applied to the object.
In a second phase P12, after a time period TE/2 from the peak of the first echo signal, a 180.degree. pulse is applied to the object. Further, after a time period TE/2 from the application of the 180.degree. pulse (i.e., after a time period TE from the peak of the first echo signal), a read gradient field Gr is applied to the object so that the peak of a second echo signal appears at this time. The second echo signal is acquired in the state wherein the read gradient field Gr is being applied to the object.
The above sequence is repeated such that the amplitude of the encoding gradient field Ge applied between the 90.degree. pulse and 180.degree. pulse in the first phase P11 is varied by a predetermined value in every encoding step.
Third and following echo signals can be acquired in the following manner. As in the case where the second echo signal is acquired in the second phase, a 180.degree. pulse is repeatedly applied after the time period TE/2 from the peak of the previous echo signal and the read gradient field Gr is applied to the object. Thus, an echo signal, the peak of which appears after the time period TE/2 from the application of the 180.degree. pulse, is acquired.
According to the above-described multiecho sequence data acquisition method, the intensities of the second and following signals gradually lower, and the S/N is inevitably lowered. However, a number of MR signals (MR data) can be obtained by the execution of a single sequence. For example, a plurality of atomic nucleus density distribution images having different emphases of relaxation time constant information can be obtained. Thus, this acquisition method is advantageous in clinical examinations.
However, in the above-described multiecho sequence MR data acquisition method, because of the degradation in S/N in the second and following echo signals, the influence due to movement of atomic nuclei is not negligible, and artifact in images due to the second and following echo signals becomes considerable. Consequently, the definition of image is degraded, and it becomes difficult to exactly distinguish, for example, a normal tissue from a diseased part.